s390/smsgiucv_app: Make use of iucv_alloc_device()

s390/smsgiucv_app: Make use of iucv_alloc_device()

Make use of iucv_alloc_device() to get rid of quite some code. In addition
this also removes a cast to an incompatible function (clang W=1):

  drivers/s390/net/smsgiucv_app.c:176:26: error: cast from 'void (*)(const void *)' to 'void (*)(struct device *)' converts to incompatible function type [-Werror,-Wcast-function-type-strict]
    176 |         smsg_app_dev->release = (void (*)(struct device *)) kfree;
        |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/r/20240417-s390-drivers-fix-cast-function-type-v1-2-fd048c9903b0@kernel.org
Acked-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://lore.kernel.org/r/20240506194454.1160315-5-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>